The present invention relates to a method of producing a fuel cell separator, a fuel cell separator, and a polymer electrolyte fuel cell. In particular, the present invention relates to a method of producing a fuel cell separator at a reduced rate of occurrence of molding failures, accordingly, at a high efficiency, a high quality fuel cell separator produced by the method, and a polymer electrolyte fuel cell including a plurality of unit cells whose separators are partly or entirely composed of the fuel cell separators, which cell has a high gas sealing characteristic and an excellent impact resistance, and is particularly suitable as a movable power supply for automobiles, small-sized marine vessels, etc.
A fuel cell, particularly, a polymer electrolyte fuel cell, is configured as a cell stack formed by juxtaposing unit cells of the number of several tens to several hundreds, wherein each of the unit cells includes, as shown in FIG. 1, a pair of fuel cell separators 1 each having on its right and left side surfaces a plurality of ribs 1a, and a polymer electrolyte membrane 2 and two gas diffusion electrodes 3 interposed between these separators 1.
As shown in FIG. 2, the fuel cell separator 1 has a unique shape having the plurality of ribs 1a projecting from the right and left side surfaces of a thin plate-like body, wherein passages (grooves) 4 for supplying and discharging a fuel gas such as hydrogen or oxygen are formed between the ribs 1a on each side of the separator 1 and the corresponding electrode 3. Accordingly, the separator 1 is required to have a high elasticity, an excellent dimensional accuracy, and a desirable gas non-permeability, and the unit cell (fuel cell) is required to have a high gas sealing characteristic capable of preventing a leak of a fuel gas, and an excellent impact resistance, particularly, when the fuel cell is used as a movable power supply for automobiles, etc.
Such a fuel cell separator has been produced by various methods, for example, a method (1) of forming a plate-like body from phenol resin, polyimide resin, or furan resin by molding, hardening and baking the resin of the plate-like body, and forming ribs by machining; a method (2) of impregnating a dense carbon shaped into a plate-like body with a thermosetting resin, and forming ribs by machining; a method (3) of impregnating carbon fiber unwoven fabrics with a conductive paint prepared by adding a solvent and a conductive filler to a thermosetting resin such as epoxy resin or phenol resin, and laminate-molding the resultant fabrics into the shape of a fuel cell separator; and a method (4) of kneading a carbon powder having a particle size of 100 mesh or less with phenol resin, and hot-press molding the kneaded material into the shape of a fuel cell separator.
Each of the methods (1) and (2), however, has problems that since the ribs are formed by machining, the cost is correspondingly raised as well as the number of steps is increased, and that the separator, if it is of a thin type, is liable to be cracked during machining or assembly of a fuel cell.
The method (3) has a problem that the ribs, that is, the grooves of the separator cannot be accurately formed by laminate-pressing because of non-flexibility of the unwoven fabrics.
The method (4) has a problem that since the mixture of the carbon material and the resin is molded, if the content of the carbon material as a conductive filler is made large for improving the conductivity, the moldability and the mechanical strength are degraded, and if the content of the binder resin is made large for improving the moldability and the mechanical strength, the conductivity is lowered because of the small content of the carbon material.
By the way, since a voltage outputted from each unit cell of a fuel cell is low, the fuel cell is required to have an array of unit cells of the number of several tens to several hundreds for ensuring a practical output ( less than several hundreds kW). Therefore, to uniformly arrange a large number of unit cells in parallel, it is required to establish a technique of producing fuel cell separators, each of which is formed into a uniform shape with no deformation and no partial unevenness in thickness, on a large scale with a high yield without occurrence of molding failures such as chips, cavities, and the like.
The conventional method of producing a fuel cell separator mainly containing a thermosetting resin such as phenol resin and graphite, however, cannot satisfy the above-described requirement. The reason for this is as follows. The conventional method generally involves packing a compound in a separator mold having a specific shape, and hot-press molding the compound for 5 to 10 min at a temperature of 150 to 160xc2x0 C. and a pressure of 10 to 50 MPa. Such a method has a problem, from the viewpoint of uniformly producing fuel cell separators with a high quality, that since the compound contains a large amount of graphite for imparting a necessary conductivity, the flowability of the compound is poor and accordingly it is difficult to uniformly pack the compound in the separator mold, to cause a large number of molding failures. The conventional method has another problem that molding failures may occur due to a gas generated at the time of hot-press molding.
An object of the present invention is to provide a method of producing a fuel cell separator at a reduced rate of occurrence of molding failures, accordingly, at a high efficiency, a fuel cell separator produced by the method and having a high dimensional stability, an excellent gas non-permeability, and a uniform quality, and a polymer electrolyte fuel cell including a plurality of unit cells whose separators are partly or entirely composed of the fuel cell separators, which cell has a high gas sealing characteristic and an excellent impact resistance, and is particularly suitable as a movable power supply for automobiles, small-sized marine vessels, etc.
To achieve the above object, the present inventor has examined to improve a method of producing a fuel cell separator. The production method includes the steps of producing dry granules of a composition for a fuel cell separator mainly containing a conductive material, a binder, and an additive by mixing raw materials of the composition, granulating the mixture, and drying the granules; and packing the dry granules in a mold, and hot-press molding the dry granules. With respect to such a method, the present inventor has found that, to produce a high quality fuel cell separator at a high production yield, it is effective to adjust an average particle size and particle size distribution of the dry granules in respective specific ranges and to reduce a residual volatile matter content of the dry granules.
The findings by the present inventor will be described in detail with reference to FIG. 3. FIG. 3 is a graph showing a particle size distribution of dry granules of a composition for a fuel cell separator, produced by mixing, granulating and drying. In this graph, a curve (B) shows a sharp particle size distribution of a granule sample A, and a curve (A) shows a gentle particle size distribution of a granule sample B. In the case of packing each of the granule samples A and B in a separator mold, even if the average particle size of the granule sample A is equal to that of the granule sample B, the degree of occurrence of voids of the granule sample A is larger than that of the granule sample B, and therefore, it is difficult to uniformly pack the granule sample A as compared with the granule sample B. That is to say, it has been found that the use of granules having a suitable average particle size and having a wide particle size distribution, or the use of fine granules most of which have particle sizes smaller than 500 xcexcm, produced by sizing the granules obtained by mixing, granulating and drying, is effective to improve the flowability and the packing rate of the granules and hence to suppress the occurrence of molding failures. Further, it has been found that the reduction in a residual volatile matter content of the granules is effective to make the generation of gas at the time of molding as small as possible and hence to suppress the occurrence of molding failures.
As a result of further examination of the production method on the basis of the above-described knowledge, the present inventor has found the following fact. Namely, granules having a residual volatile matter content of 4 parts by mass or less, an average particle size in a range of 200 to 700 xcexcm, and a particle size distribution (5 to 30% of particles: 5 xcexcmxe2x89xa6D (particle size)  less than 100 xcexcm, 10 to 40% of particles: 100 xcexcmxe2x89xa6D less than 300 xcexcm, 10 to 50% of particles: 300 xcexcmxe2x89xa6D less than 500 xcexcm, and balance: 500 xcexcmxe2x89xa6D less than 1000 xcexcm) are obtained by adding and mixing the additive to and with the conductive material, to obtain a sub-mixture, adding the binder to the sub-mixture in an amount of 5 to 30 parts by mass on the basis of 100 parts by mass of the conductive material and simultaneously adding a solvent to the sub-mixture in an amount of 20 parts by mass or less on the basis of 100 parts by mass of the conductive material, and wet-mixing the sub-mixture with the binder and the solvent; or fine granules most of which have sizes in a range of less than 500 xcexcm are obtained by further sizing the granules obtained by the above production method. The present inventor has further found that, by the use of the above granules or fine granules, it is possible to significantly improve the flowability and the packing rate of the granules and hence to uniformly pack the granules or fine granules in a separator mold having a significantly complicated groove structure, and to significantly reduce the generation of gas at the time of molding, and therefore, it is possible to produce a fuel cell separator having a high dimensional stability and a high quality at a very low rate of occurrence of molding failures such as chips or cavities.
The present inventor has also examined to improve a polymer electrolyte fuel cell, and found that a polymer electrolyte fuel cell which has an excellent gas sealing characteristic and a high impact resistance, and which is particularly suitable as a movable power supply for automobiles, small-sized marine vessel, etc. is obtained by using, as part or all of separators of unit cells of the fuel cell, the above-described fuel cell separators having an excellent gas non-permeability and a high quality without occurrence of molding failures.
Accordingly, a first aspect of the present invention, there is provided a method of producing a fuel cell separator, including the steps of: producing dry granules of a composition for a fuel cell separator mainly containing a conductive material, a binder, and an additive by mixing raw materials including at least the conductive material, the binder, and the additive, granulating the resultant mixture to obtain granules, and drying the granules; and packing the dry granules in a mold, and hot-press molding the dry granules packed in the mold; wherein a residual volatile matter content of the dry granules is in a range of 4 wt % or less; an average particle size of the dry granules is in a range of 200 to 700 xcexcm; and a particle size distribution of the dry granules is as follows:
According to a second aspect of the present invention, there is provided a method of producing a fuel cell separator, including the steps of: producing sized dry granules of a composition for a fuel cell separator mainly containing a conductive material, a binder, and an additive by mixing raw materials including at least the conductive material, the binder, and the additive, granulating the resultant mixture to obtain granules, drying the granules, and sizing the dry granules; and packing the sized dry granules in a mold, and hot-press molding the sized dry granules packed in the mold; wherein a residual volatile matter content of the sized dry granules is in a range of 4 wt % or less; an average particle size of the sized dry granules is in a range of 60 to 160 xcexcm; and a particle size distribution of the sized dry granules is as follows:
According to a third aspect of the present invention, there is provided a method of producing a fuel cell separator, including the steps of: producing dry granules of a composition for a fuel cell separator mainly containing a conductive material, a binder, and an additive by mixing raw materials including at least the conductive material, the binder, and the additive, granulating the resultant mixture to obtain granules, and drying the granules; and packing the dry granules in a mold, and hot-press molding the dry granules packed in the mold; wherein the step of mixing raw materials including at least the conductive material, the binder, and the additive comprises the steps of: adding and mixing the additive to and with the conductive material, to obtain a sub-mixture; adding the binder to the sub-mixture in an amount of 5 to 30 parts by mass on the basis of 100 parts by mass of the conductive material and simultaneously adding a solvent to the sub-mixture in an amount of 20 parts by mass or less on the basis of 100 parts by mass of the conductive material; and wet-mixing the sub-mixture with the binder and the solvent.
According to a fourth aspect of the present invention, there is provided a fuel cell separator produced by any one of the production methods according to the first, second, and third aspects of the present invention, wherein a gas permeability of the separator, measured by a method B specified under JIS K7126, is in a range of 30 cc/m2xc2x724 hrxc2x7atm or less.
According to a fifth aspect of the present invention, there is provided a polymer electrolyte fuel cell including: a plurality of unit cells juxtaposed, each of which has a pair of electrodes disposed with a polymer electrolyte membrane put therebetween, and a pair of separators disposed with the electrodes put therebetween and shaped to form gas supply/discharge passages between the separators and the electrodes; wherein the fuel cell separators according to the fourth aspect of the present invention are used as part or all of the separators in the fuel cell.